I read "Seven Habits" and used the Franklin-Covey method for several years before becoming aware of David Allen and learning the GTD methodology. They are different approaches to the same issue of taking control of and managing your life. Although I have converted to GTD and use OF to put it into action in my life, there are several FC ideas that I still embrace and feel are compatible with GTD - the four quadrant concept being one of them. As ksrhee stated above, I also use flags, due dates, and a special perspective to keep my tasks consistent with the four quadrants. Here's what I do:
First, you have to use flags judiciously to denote "important" tasks. You cannot get carried away with flagging so many tasks that importance loses its significance.
Second, you have to use due dates pretty much as David Allen says - only for true "hard" deadlines. You cannot use due dates to mean when you would "like" to complete a task without diluting the real meaning of "due."
Third, the main perspective that I use for actually "doing" is one that I call "All Available." I did not set this up to reflect the four quadrants, but I realized after I started using it that it actually separates my tasks into the four Covey quadrants. This perspective is a context view mode perspective, and the view bar settings are: Active, Flagged, Due, Available, Any, Any. What it shows me is this:
The perspective has two sections: Flagged (Important) followed by Unflagged (Not Important). At the top of the list in the Flagged section are those (important) tasks that have due dates (Urgent) with the most overdue/urgent task first on down to the task with the least urgent due date. These are Quadrant I tasks (Urgent and Important). The rest of the flagged section are those tasks that do not have due dates. They are Quadrant II tasks (Important but not Urgent). Then the Unflagged section follows with tasks with due dates first, again with the most overdue/urgent task first on down to the task with the least urgent due date. These are Quadrant III tasks (Urgent but Not Important). The rest of the tasks in the second section do not have due dates. They are Quadrant IV tasks (Not Urgent and Not Important).
Now, I must say that my definitions are modified somewhat from Stephen Covey's because his definition of Q4 tasks (for instance) are that they are worthless. I view my four groupings as relative to each other, because hopefully the truly worthless tasks that I had were deleted from OF. But there are times that I review the Q4 tasks and decide to delete some of them because I am just not going to get to them. The same thing occasionally occurs with my Q3 tasks. As I review my list in my "doing" mode, I pay the most attention the the flagged tasks (Q1 & Q2) and then look down at the top of the unflagged section (Q3). Those are the tasks that I am normally thinking about doing. I only go farther down on rare occasion. So, for me, this perspective matches both my relative "priorities" and the Franklin-Covey four-quadrant philosophy.
Hope that helps!